1. Field of the Invention
The present invention relates to a high pullulan content product, and its preparation and uses; more particularly, to a high pullulan content product containing pullulan having an average molecular weight less than 250,000, said product being prepared by allowing a microorganism capable of producing pullulan to continuously cultivate in a nutrient culture medium containing a 10-20 w/v % saccharide while controlling the viscosity of said nutrient culture medium to a level below 30 centipoise (the wording "centipoise" will be abbreviated as "cp" hereinafter), as well as to the preparation and uses of said product.
2. Description of the Prior Art
Pullulan, a viscous glucan which is prepared by allowing a microorganism capable of producing pullulan to cultivate under aerobic conditions in a nutrient culture medium containing a saccharide such as mono- and oligo-saccharides, has been industrially produced.
Examples of such a preparation of pullulan which have been proposed are a method as disclosed in Japanese Patent Laid-Open No.42,894/74 wherein proposed is a method to increase the pullulan productivity by adjusting the initial pH of a nutrient culture medium to pH 5.5-8.0, preferably, to a relatively-high level of pH; and a method as disclosed in Japanese Patent Laid-Open No.130,993/77 wherein proposed is a method to increase the pullulan productivity by culturing a microorganism capable of producing pullulan while preventing the decrease of the viscosity of a nutrient culture medium by coexisting an amylase-inhibitory substance. It was found that these methods, however, are batchwise cultures so that they are not necessarily satisfiable in view of the industrial-scale production of pullulan.
The following publications disclose a preparation of pullulan prepared by a continuous culture of a microorganism capable of producing pullulan:
(1) Applied Microbiology and Biotechnology, Bulmer et at., Vol.25, pp.362-365 (1987); PA1 (2) Biotechnology and Bioengineering, Boa et al., Vol.30, pp.463-470 (1987); PA1 (3) Biotechnology and Bioengineering, McNeil et al., Vol.33, pp.1210-1212 (1989); and PA1 (4) Enzyme and Microbial Technology, McNeil et al., Vol.12, pp.521-526 (1990).
Preparations of pullulan by using a continuous culture, however, were biochemically and biotechnologically studied in these publications, the concentration of a saccharide such as glucose, sucrose and peat hydrolyzate was low as 2-5 w/v %, and the influence of the viscosity of a nutrient culture medium was not studied. Boa et al. did not clearly refer to the pH of a nutrient culture medium, while Bulmer et al. effected a continuous culture at pH 5.5, and McNeil et al. reported that the optimum pH of a pullulan-producing microorganism was pH 4.5. Among these continuous cultures, the maximum pullulan productivity per L and per an hour (gL.sup.-1 h.sup.-1) (hereinafter will be referred to as "pullulan productivity") was low as 0.48 as disclosed in the publication of Boa et al., and the cultivation periods of the continuous cultures are not clear. Accordingly, in accordance with these publications it seems to be substantially difficult to produce a high pullulan content product which is advantageously useful in an industrial-scale production.
It has been a great demand to overcome the drawbacks of the above conventional methods and to establish a high pullulan content product which is prepared by a novel and industrially advantageous continuous-culture, as well as to establish its uses.